PROJECT SUMMARY Salmonella serovars can cause severe extraintestinal disease in humans. While neutrophils can kill extracellular bacteria using NADPH oxidase-dependent killing mechanisms, macrophages support growth of the pathogen in tissue. The main virulence factor promoting bacterial survival at extraintestinal sites, a type III secretion system (T3SS-2) encoded by Salmonella pathogenicity island (SPI)-2, functions in evading NADPH oxidase-dependent killing by host phagocytes. Paradoxically, T3SS-2-deficient mutants can grow within macrophages in the liver and spleen of mice, suggesting that the virulence factor does not evade NADPH oxidase-dependent killing by macrophages in vivo. Our central hypothesis is that entrapment of S. Typhimurium within the cellular debris of a dead macrophage (termed the pore-induced intracellular trap or PIT) protects the pathogen from neutrophil NADPH oxidase-dependent killing mechanisms during efferocytosis of cellular debris containing viable bacteria. Through this mechanism, T3SS- 2-dependent macrophage PIT formation enables S. Typhimurium to evade NADPH oxidase-dependent killing by neutrophils when the pathogen exits from infected cells to form new infection foci. We will test key aspects of our hypothesis by determining whether localization in macrophage PITs protects Salmonella from neutrophil ROS (Specific Aim 1) and by elucidating how Salmonella T3SS-2 enables complement to reach intracellular bacteria in PITs (Specific Aim 2). Successful completion of the proposed experiments will establish the novel concept that macrophage PIT formation is a virulence strategy of intracellular pathogens to evade neutrophil-mediated host defenses.